In semiconductor device assembly, a semiconductor chip (or "die") may be bonded directly to a packaging substrate, without the need for a separate leadframe or for separate I/O connectors (e.g. wire or tape). Such chips are formed with ball-shaped beads or bumps of solder affixed to their I/O bonding pads. During packaging, the chip is "flipped" onto its active circuit surface in a manner forming a direct electrical connection between the solder bumps of the chip and conductive traces on a packaging substrate. Semiconductor chips of this type are commonly referred to as "flip chips", and are advantageously of a comparatively reduced size.
Briefly, as shown in FIG. 1, a conventional semiconductor flip chip package 10 is illustrated in which a semiconductor die 11 and a packaging substrate 12 are electrically connected and mechanically bonded. The semiconductor die 11 includes an active circuit surface 13 on which are arranged solder balls 15. The solder may be composed of a low melting point eutectic material or a high lead material, for example. It should be noted that this figure is intended to be representative and, does not show the solder balls 15 in proportion to the semiconductor die 11. In current designs, the die may have dimensions on the order of 0.5.times.0.5 inch whereas the unbonded solder balls may have a diameter on the order of 4 to 5 mils.
Prior to bonding the die 11 to a substrate, solder flux is applied to either the active circuit surface 13 of the die 11 or the packaging substrate surface. The flux serves primarily to aid the flow of the solder, such that the solder balls 15 make good contact with traces on the packaging substrate. It may be applied in any of a variety of methods, including brushing or spraying, or dipping the die 11 into a thin film, thereby coating the solder balls 15 with flux. The flux generally has an acidic component, which removes oxide barriers from the solder surfaces, and an adhesive quality, which helps to prevent the die from moving on the packaging substrate surface during the assembly process.
After the flux is applied, the die 11 is aligned with and placed onto a placement site on the packaging substrate 12 such that the die's solder balls 15 are aligned with electrical traces (not shown) on the substrate 12. The substrate is typically composed of a laminate or organic material, such as fiber glass, PTFE (such as TEFLON.TM., available from Gore, Eau Claire, Wis.) BT resin, epoxy laminates or ceramic-plastic composites. Heat (to a temperature of about 220.degree. C., for example) is applied to one or more of the die 11 and the packaging substrate 12, causing the solder balls 15 to reflow and form electrical connections between the die 11 and the packaging substrate 12. Then, the remaining flux residue is substantially removed in a cleaning step, for instance by washing with an appropriate solvent.
At this point, the mechanical bonding procedure can begin. An underfill material, typically a thermo-set epoxy 16, such as is available from Hysol Corporation of Industry, California (product numbers 4511 and 4527), Ablestik Laboratories of Rancho Domingo, Calif., and Johnson Matthey Electronics of San Diego, Calif., is dispensed into the remaining space 17 (or "gap") between the die 11 and the substrate 12. In a typical procedure, a bead of thermo-set epoxy, is applied along one edge of the die at atmospheric pressure where it is drawn under the die through capillary action until it completely fills the gap between the die and the packaging substrate. Slight heating of the packaging substrate after dispensing of the underfill epoxy assists the flow.
After the epoxy 16 has bled through the gap 17, a separate bead of epoxy (not shown) may also be dispensed and bonded around the perimeter of the die 11. Thereafter, the epoxy (both the underfill and perimeter bonding epoxy, if any) are cured by heating the substrate and die to an appropriate curing temperature, for example, about 130.degree. C. to about 165.degree. C. In this manner the process produces a mechanically, as well as electrically, bonded semiconductor chip assembly, with the underfill material 16 allowing a redistribution of the stress at the connection between the die 11 and the packaging substrate 12 from the solder joints 15 only to the entire sub-die area. FIG. 1 illustrates the semiconductor die 11 interconnected to the packaging substrate 12 electrically by solder joints 15 and mechanically by a cured layer of epoxy 16.
While this conventional assembly process is adequate in many instances, several drawbacks are inherent with this design. For example, the entire flip chip assembly process includes a number of separate steps, each of which take several seconds to several minutes to perform. As noted, the step of dispensing epoxy and allowing it bleed under the gap between the die active circuit surface and the substrate alone requires several minutes. The prolonged process time reduces the throughput of the assembly process, thus limiting the production capacity and the commercial competitiveness of flip chip devices.
For a given die/package combination, the underfill epoxy flow conditions may be optimized to minimize process time, but the dynamics of the underfill epoxy flow within the gap are very sensitive to the topology of the traces on the packaging substrate and of the arrangement of solder connections to the die. Thus, optimization of the underfill flow conditions would have to be repeated for each device design which uses even a slightly different I/O configuration. Unfortunately, such process-to-process optimization generally is prohibitively time-consuming and expensive.
Another serious concern in the above-described flip chip device assembly process is the danger of voiding during the flow of the underfill epoxy in the gap between the die and the packaging substrate (i.e., the formation of spaces within the gap into which the underfill epoxy does not flow). Due to the wall effects and internal shear stress of the underfill material, the filling process often causes voids of trapped air and moisture to form therein.
As set forth above, the underfill epoxy is intended to impart mechanical strength to the chip device. Unfortunately, upon repeated thermal cycling between room temperature and .about.100.degree. C. and due to the greater coefficients of expansion, these voids of entrapped air and moisture expand and contract at rates different than the surrounding underfill material. Accordingly, the formation of stress concentrations can cause delamination of the layered chip which may ultimately propagate throughout the whole chip.
One factor which contributes to voiding is flux residue remaining in the gap between the die 11 and the packaging substrate 12 after reflow. Any such remaining flux residue may prevent wetting by the underfill epoxy 16 within the gap 17 and so must be removed prior to the dispense and flow of the underfill epoxy. Further, the acidic components of the flux, if allowed to remain on the surface of the die and/or the packaging substrate, may later corrode the flip chip device, and so the flux residue must be removed for that reason as well.
Typically, flux is removed by applying a solvent to the chip device at high pressures. Unfortunately, such solvent is commonly a hydrocarbon-based liquid having a high flash point and high toxicity, creating numerous safety concerns and environmental problems associated with waste solvent disposal.